Many integrated circuits include a metal-oxide-semiconductor field-effect transistor, or “MOSFET” for short, which includes a gate, a source, a drain, and a body. An issue in MOSFET design involves the structure and composition of its gate. Some early MOSFET designs included aluminum gates, and later MOSFET designs used polysilicon gates because of the desire for a self-aligned gate, the tendency of aluminum to diffuse through the underlying insulative layer, and because of problems that the relatively low melting temperature of aluminum caused with annealing processes. Polysilicon can be doped to act as a conductor, but with significantly more electrical resistance than aluminum. This higher resistance can be ameliorated somewhat by silicidation. However, the higher resistance of even the salicided polysilicon gates combines with inherent integrated-circuit capacitances to cause significant delays in conducting signals from one circuit point to another, ultimately limiting how fast integrated circuits operate.
Currently, the semiconductor industry relies on the ability to reduce or scale the dimensions of the basic components, including the gate dielectric, of its transistor devices to obtain lower power consumption and higher performance. To reduce transistor size, the thickness of the gate dielectric is reduced in proportion to the shrinkage of the gate length. Increased scaling and other requirements in microelectronic devices have created the need to use other dielectric materials as gate dielectrics, in particular dielectrics with higher dielectric constants (k) to replace the conventional use of various combinations of SiO2, Si3N4 and SiON. Practical higher dielectric constant (k) materials have the properties of high permittivity, thermal stability, high film and surface quality and smoothness, low hysteresis characteristics, low leakage current density, and long term reliability. However, polysilicon gates and high-k dielectric materials have interface instability issues.
Scaling of transistors also requires shallow, difficult-to-form, source/drain extensions. In some conventional processes, source/drain extensions are formed, and then sidewall spacers are used to define the source/drain regions. The subsequent high-dose implant and high-temperature anneal of the source/drain regions can negatively impact the lightly doped source/drain extensions.